The cause(s) of idiopathic pulmonary fibrosis (IPF) remain unknown, as well as the factors that result in the progression of this disease. No established medical treatments have yet been shown to benefit patients with this disease, and IPF continues to have a worse prognosis than many common malignancies. Recent findings of our research group, as well as others, show that autoantibodies are associated with IPF progression. Results of an early ongoing pilot study further indicate that specific treatments aimed at reducing pre-existing autoantibodies and/or minimizing their future production improve lung function among very ill IPF patients who are having disease exacerbations. We hypothesize that antibody-mediated autoimmunity can play an important role in IPF progression. The presence of autoimmunity could explain the refractoriness of this disease to current therapy, since many autoantibody lung diseases are resistant to treatment with corticosteroids. However, focused treatments targeted at autoantibodies or the lymphocytes that produce these immunoglobulins often have greater efficacy for these diseases than steroids or other nonspecific therapies. Accordingly, we propose here a multicenter, randomized, double-blind, placebo-controlled Phase II clinical trial to explore the efficacy and safety of rituximab (anti-CD20 monoclonal antibody) vs. placebo, among patients with IPF. Fifty-eight (58) ambulatory IPF subjects at four participating U.S. medical centers will be randomized (1:1) to either the experimental arm (rituximab) or placebo. The primary end-point is a global assessment of circulating autoantibodies. Secondary end-points are measures of a specific autoantibody that is associated with clinical progression (anti-heat shock protein 70), as well as a measure of lung function (forced vital capacity), and adverse event rates. We anticipate the novel experimental treatment will show targeted efficacy for reduction of autoantibodies, a favorable safety profile, and possibly stabilization of lung function. Results of these investigations will establish the efficacy and pharmacokinetics of rituximab treatment in IPF patients. Ultimately these studies could challenge current paradigms of IPF pathogenesis, and substantially alter treatment approaches to patients afflicted with this morbid, refractory disease.